Thermal cracking of hydrocarbons is a large-scale process for the production of light olefins, such as ethylene and propylene, which are major building blocks of the petrochemical industry. Referring to FIG. 1, a portion of a thermal cracking process is schematically illustrated. Feedstock, such as naphtha, methane, ethane, propane, or butane, is cracked in a pyrolysis or cracking furnace (not shown) to generate light hydrocarbons. The process gas leaves the furnace at temperatures ranging from 750 to 900° C. (1400 to 1650° F.) and at pressures between 0.5 to 1.0 bar (7 to 15 psig). The products in the process gas leaving the furnace are not stable at the high temperature at the outlet of the furnace. To avoid overreactions and loss of light olefins, the process gas is rapidly cooled after leaving the furnace in a number of quenching stages, which quickly stop the chemical reactions of the process gas.
An initial quenching stage uses a transfer line heat exchanger 30 known in the art. The transfer line heat exchanger 30 is a tube and shell type heat exchanger that is cooled by feed water steam as an intermediate heat carrier. Piping 20 connects the source of the process gas (e.g., the furnace) to the transfer line heat exchanger 30. Typically, the transfer line heat exchanger 30 is much larger in diameter than the piping 20 used to convey the process gas so that an inlet section 22 is typically used to expand the piping 20 to fit the larger diameter of the transfer line heat exchanger 30.
The transfer line heat exchanger 30 includes a shell 32, a plurality of heat transfer tubes 34, an inlet tube sheet 36, and an outlet tube sheet 38. An inlet 40 and an outlet 42 for feed water connect to the shell 32. The heat exchanger 30 may contain as many as 1500 to 2000 transfer tubes 34 through which the process gas flows from an inlet section 22 to an outlet section 24. The transfer tubes 34 are connected to holes in the tube sheets 36 and 38. Tie rods and baffle plates within the shell 32 are used with the bundle of tubes 34. As the tubes 34 carry the gas through the shell 32 of the heat exchanger 30, the tubes 34 are surrounded by feed water steam that flows through the shell 32 of the heat exchanger 30 for cooling the process gas. The discharge section 24 connects to additional piping 26 of the process system, where the process gas is taken to for continued processing, such as further quench stages to cool the gas. When quenching the process gas during use, the stream of process gas after leaving the furnace of the industrial cracker may be cooled within the heat exchanger 30 from 850° C. or more, down to 400° C. or less.
Referring to FIG. 2, the inlet section 22 and the inlet tube sheet 36 of the transfer line heat exchanger are schematically shown. The inlet section 22 and tube sheet 36 are symmetric about a central axis C so that only a portion of the inlet section 22 and tube sheet 36 is shown for convenience. The inlet section 22 is typically lined with refractory 40 for thermally insulating the inlet section 22. Streamlines G schematically show the laminar flow of the process gas as it travels from the furnace piping 20, to the inlet section 22, through the holes in the tube sheet 36, and into the transfer tubes 34. As noted above, the tube sheet 36 typically has a number of transfer tubes connected to holes 37 in the tube sheet 36. Only a few tubes 34 are illustrated for convenience.
As is known in the art of chemical processing, the transfer line heat exchanger 30 such as described herein can suffer from a number of problems. For example, problems can occur at the inlet tube sheet 36 of the exchanger 30. In many applications, for example, a recirculation zone can occur in the inlet section 22 near the face F of the inlet tube sheet 36. The recirculation zone is schematically shown in FIG. 2 near the periphery of the inlet tube sheet 36. The geometry of the inlet section 22 along with the flow rate of the process gas creates the recirculation zone in the inlet section 22. The formation of the recirculation zone thereby increases the peak velocity at the tube sheet 38, which can be as great as 1880 in/sec, for example. Ideally, if the formation of the recirculation zone did not occur, the process gas would be more uniformly distributed at the face of the tube sheet 36, and the average velocity would be approximately 566 in/sec, for example.
Due to the recirculation, the process gas may be poorly distributed at the face of the tube sheet, and the velocity of the process gas is greater than ideally desired. Under these conditions, the heat transfer film coefficients on the face F (i.e., the process gas side) of the tube sheet 36 will be higher than ideal, and the associated temperatures will be higher than if the flow of the process gas were more evenly distributed. In addition, recirculation can cause fouling on certain portions of the tube sheet 38, for example the outer edge, so that the chances of plugging of certain tubes 34 are increased. For example, the formation of carbonaceous deposits can accumulate near the periphery of the tube sheet 36, diminishing the ability of the process gas to pass through the outer tubes 34. Such fouling conditions decrease the efficiency of the system.
If a group of tubes 34 becomes plugged from the recirculating gas, then the peak velocity near the open portion of the tube sheet 36 will further increase, creating jetting conditions or a jetting zone in the piping 20 and inlet section 22. The high velocity gas streams in the jetting zone can produce film boiling on the tube sheet 36. As is known in the art, film boiling can cause the welded joint of the tubes 34 to the sheet to fail and can exacerbate corrosion at the welded joint. In addition, erosion of the tube sheet 36 can occur if there are particles in the process gas, and such erosion can be further compounded if jetting conditions are produced.
To reduce the problem of fouling, periodic removal of the fouling deposits may be necessary. Typically, the transfer line heat exchanger 30 must be put out of service to remove the fouling. In the art, coatings may also be used to reduce the potential for fouling. Unfortunately, the intense heat of the process gas in the inlet section 22 can quickly destroy any such coatings. To solve problems related to recirculation and jetting conditions, it is known in the art to form the refractory 40 in the inlet section 22 in a shape that can reduce the recirculation of the process gas near the edge of the tube sheet 36. For example, the refractory 40 may be given a “bell” or “trumpet” shape from the piping 20 to the face F of the tube sheet 36. Other solutions in the prior art include inserting a piece of equipment to breakup the gas flow in the inlet section 22. Because the heat of the process gas is so intense, the equipment used to break up the flow can be quickly destroyed, which can lead to additional problems. To prevent erosion, another device known in the art, called an “Erosion Protection Shield” manufactured by Borsig Gmbh, is placed in the inlet section.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.